The present invention relates in general to a control system for an electric motor vehicle driven by induction motors and in particular to a control system for improving the adhesion performance of the electric motor vehicle.
In the electric motor vehicle driven by induction motors, it has been hitherto known that the speed control of the electric motor vehicle is effected by regulating the frequency of an a.c. current supplied to the induction motors. For the frequency regulation or adjustment, the varying rate of the frequency has to be selected from a range of acceleration and deceleration permissible for the induction motors so that the acceleration as well as the deceleration of the electric motor vehicle can be accomplished smoothly. To this end, a frequency control-command is produced by detecting the number of revolutions of an induction motor and adding to the detected number of revolutions a value corresponding to a slip frequency at the time when the acceleration is controlled in the power running or subtracting the same value from the detected number of revolution when the control for deceleration is to be performed in the regenerative braking mode. When the control command thus prepared is utilized for the speed control, the frequency of the a.c. current supplied to the given induction motor is prevented from being increased (in the power running mode) or decreased (in the regenerative braking mode) independently from the number of revolutions of the induction motor. As a consequence, the electric motor vehicle can be controlled smoothly in respect of acceleration and deceleration. However, in the event that the wheels driven by the given induction motor undergo slipping (in the power running mode) or sliding (in the regenerative braking mode), the frequency of the a.c. current supplied to the induction motor will be increased (in the case of wheel slip) or decreased (in the case of wheel slide) following wheel slip or slide, resulting in the disadvantages promotion of wheel slip or slide.
A system for driving an electric motor vehicle by means of the induction motors is reported in a literature "ZEV-Glas. Ano. 101" (1977), Nr. 8/9, August/September, page 363 to 370, for example. However, this prior art neither teaches nor suggests any means for improving the adhesion characteristic or performance of the electric motor vehicle.